The present invention relates to windows for the passage of desired acoustic waveforms, and specifically to such windows employed in submerged liquid service such as underwater oceanic service. More particularly, the invention relates to sonar windows such as domes for use on surface and submergible vessels in both the military and commercial arenas.
Acoustic windows such as sonar domes for use in transmitting or receiving acoustic waveform signals in a liquid environment are well known in the art. Typically, these windows have consisted of a single thickness of a fiberglass composition optionally covered by a coating substance to minimize the fouling of surfaces of the window.
Typically, the exterior surface of such windows is exposed to a body of free liquid such as an ocean, lake or tank. The interior surface of such windows conventionally has at least partially defined a chamber filled with water or another liquid. In the prior art designs, it was important in submerged liquid applications to build windows to withstand a particular structural loading. Secondarily was a consideration to configure such windows to be acoustically xe2x80x9cclear.xe2x80x9d That is, the window would have a desirably low distortion and attenuation of sound wave energy passing through the windows, as well as exhibiting a desirably low distortion of the angle characterizing the impingement of the wave energy against the window. However, the use of rigid high strength materials, in particular to meet structural loading requirements, has tended to make xe2x80x9ctuningxe2x80x9d sonar windows formed with such materials quite difficult. The properties of the materials of construction for the sonar windows, together with the structural loading imposed upon such windows, has tended to establish the acoustic properties of the window without much residual flexibility for tuning of the properties such as transmission loss and the like.
Windows such as sonar domes, depending of course on the particular application, can be required to transmit acoustic energy having a frequency ranging from about 10 Hz to about 1.5 MHz. These frequencies correspond to wavelengths of from about 150 meters to about 0.001 meters in water, respectively, with the wavelengths being subject to some variation depending upon the material through which the waveform is being propagated. However, many known prior art acoustic windows are limited to use in a certain frequency range due to the above-described structural and acoustical choices made in design of the windows.
Of course, it is understood that sonar domes are not the sole use for acoustically transparent materials. Frequently, it is desired that acoustic waveform energy be transmitted through a generally flush window or covered aperture in a vessel hull. The same constraints that limit use of conventional sonar domes to a certain frequency range also limits the use of such windows.
A number of efforts have been made to develop a sonar window, tunable to substantially reduce sound wave attenuation or distortion upon passage through the window, as well as sonar windows which reduce the reflective signals during passage of an acoustic waveform signal, by forming them from a plurality of materials. For example, U.S. Pat. No. 4,997,705 to Caprette, Jr. et al., teaches a laminate acoustic window for sonar systems having a pair of septa sandwiching a core, where the core is made of a low shear high elongation-to-break material and the septa are formed of a high modulus material. The windows of the invention are characterized by unusual freedom from attenuation loss over a wide, albeit generally low, frequency range. The windows are substantially self-damping and avoid thereby a generation of significant quantities of deleterious noise due to self-generated vibration and transmitted vibration. Caprette focuses on normal incidence angles, which is useful and broad since virtually all acoustic window applications include some normal incidence waveform transmission. In practice, however, most wave form transmissions occur at non-normal incidence angles, especially when the windows are curved. Caprette does not teach or suggest how to configure an acoustic window to achieve uniform (e.g. consistent within xc2x11 dB) transmission loss across a range of incidence angles and frequencies.
Other compositions include U.S. Pat. No. 4,770,267 to Hauser which teaches a sandwich construction where the central layer is a rigid core composed of glass fibers impregnated with resin and the peripheral layers are a plurality of woven webs of carbon fiber impregnated with thermosetting epoxy resin, but Hauser does not use or rely on the use of an elastomeric core material. U.S. Pat. No. 3,858,165 to Pegg teaches an acoustic window composition which is a laminate of layers of fiber glass cloth in an epoxy binder and which contains high strength glass microspheres or elastomeric polymer. U.S. Pat. No. 4,784,898 to Raghava teaches a composition of a layer of low loss sonar material, such as polyethylene and layers of fiberglass and epoxy.
The present invention is the result of the discovery that a composition acoustic window for an acoustic waveform passage having a generally uniform (less than about xc2x13 dB variation, preferably less than xc2x11 dB variation between +40xc2x0 and xe2x88x9240xc2x0 angles of incidence at a given frequency) non-normal acoustic performance can be achieved from a composition formed from at least one core layer and at least two septa, where the core layer is a material having a generally low-acoustic-impedance, a static shear modulus between about 1.0 psi (0.007 MPa) and about 15,000 psi (103 MPa), a transverse (or through-thickness) sound velocity for the acoustic waveform of between about 700 and about 2200 meters per second, a transverse (or through-thickness) acoustic impedance of less than or equal to 4xc3x97106 kilograms per square meter-second, and a shear loss factor of greater than 0.02 (and preferably greater than 0.1 shear loss factor), and the septa comprises at least one ply of a material which has a transverse acoustic impedance of less than 60xc3x97106 kg/m2-sec, and preferably less than 10xc3x97106 kg/m2-sec, a thickness of less than 0.10 xcexM, and is bonded to the core to form a sandwich with the core layer. xcexM is the wavelength (meters) of the acoustic wave in the material and is calculated as the sound velocity at normal incidence (meters per second) of the material divided by the frequency (hertz) of the sound. In this instance, the material is the septa. The thickness of the window is measured similarly, except the weighted average sound velocity of the window is used and it is represented by xcexW. Ideally, the thickness of the composition or the window will be less than 1.0 xcexW, with less than 0.75 xcexW being preferred.
The septa can be a material such as a plastic, a metal, or a composite material, with a carbon fiber reinforced epoxy composite being preferred. The septa have a tensile modulus of more than 0.5xc3x97106 psi to maintain the window shape under structural loading. In most designs, the septa with significantly higher tensile and compression moduli are necessary to meet structural requirements. By maintaining thin septa (preferably thinner than 0.05 xcexM), the generally uniform non-normal acoustic performance can be achieved over incidence angles of between xe2x88x9240xc2x0 and +40xc2x0, where the angle of incidence is the angle between a plane which is the tangent to the window surface and a plane which is normal to the wave propagation vector. The preferred performance can be achieved over incidence angles of between xe2x88x9260xc2x0 and +60xc2x0, with between xe2x88x9280xc2x0 and +80xc2x0 being further preferred. Acoustic windows of the present invention provide a waveform passage that can be utilized in a variety of acoustic window applications such as sonar domes and windows formed in vessel hulls.